Field of the Invention
The invention relates to a method for transmitting information from, for example, a portable apparatus to a write- and/or read module, to a system for carrying out this method, to a data-transmitting apparatus as well as a to write- and/or read module.
Description of Related Art
A multitude of channels are available for transmitting information signals, in particular digital signals, between transmitters and receivers. Such a channel is the capacitive (more precisely: capacitive/resistive) coupling between a portable apparatus and a write- and read module. The application of such a coupling is of particular interest when it is effected via the human body as a transmission medium. Corresponding systems are disclosed for example in the U.S. Pat. No. 4,591,845, the U.S. Pat. No. 5,914,701 and the U.S. Pat. No. 5,796,827. A user carries a portable apparatus with him. Information flows as soon as the user touches a touch surface coupled to a read- and write module, or is located in the direct vicinity of the touch surface. For example, an unambiguous access code may be transmitted from the portable apparatus to the write- and read module.
For practical applications, in particular for access control in its broadest sense, the following demands arise, which in combination are not met by any existing system, and which until now have prevented a commercial breakthrough of this type of information transmission:
A. Signal-to-noise ratio: A favorable signal-to-noise ratio is only possible with a large amplitude of the transmission signal. Fluctuations of potential which exist in the—high-resistance—human body and which are impressed by electrical apparatus, are of an absolute value of a few 100 mV, in the range of up to 1 MHz. A much larger signal amplitude (i.e. high voltage on the body) for a transmission system is however not tolerated by the user. The method should therefore also function with unfavourable signal-to-noise ratios.B. Inexpensive components of the portable apparatus: Whereas the simplest of passive components are sufficient for example for RFID-information transmission, a portable apparatus for capacitive transmission must comprise an active transmitter with a voltage source, and there exists the problem of synchronisation with the receiver. Precise clock generators (crystal or likewise) are however expensive, and the synchronisation effort increases with less precise clock generators.C. Speed: the complete information transmission process including synchronisation should last a few seconds at the most, even better less than one second, and at the most 300 ms or at the most 200 ms, depending on the application.
It has already been suggested in the U.S. Pat. No. 5,914,701 to apply the direct-sequence spread spectrum modulation method for information transmission. The noise sensitivity (in particular the interference sensitivity is meant by this) is reduced by way of this, and it becomes possible for several transmitters to be simultaneously active, wherein each transmitter has its own modulation code (spreading code). Indeed, the spread spectrum method which per se has been known for some time, as known, is suitable for reducing the interference-proneness of signals and for coding the signals in a receiver-specific manner. However, disadvantages also result: the computation efforts at the write- and/or read module, and the synchronisation effort at the portable apparatus, are considerable. The mentioned document U.S. Pat. No. 5,914,701 does not suggest how the synchronisation may be accomplished without compromising the demands B and C. Furthermore, depending on the application, it may also be disadvantageous if it is simultaneously possible for several portable apparatus to communicate with the write- and/or read module. For example, for the application of a “secure access control”, one should rather ensure that the data received by the write and/or read module only originates from that user who is located in the direct vicinity of an operating surface of the module, and touches this for example.